This invention relates to liquid ink printers, and more particularly to a pressure change accommodating ink container or tank for use in such a liquid ink printer.
Liquid ink printers such as ink jet recording apparatus of the type frequently referred to either as continuous stream or as drop-on-demand, be they piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. For a drop-on-demand printhead power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet recording apparatus, the power pulses are usually produced by formation and growth of vapor bubbles on heating elements or resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially expels the ink therein from the channel orifice, thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium where, upon hitting the recording medium, a dot or spot of ink is deposited. Following collapse of the vapor bubble the channel is refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet recording apparatus is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type recording apparatus, a partial width array type recording apparatus, or a page-width type recording apparatus. The carriage type recording apparatus typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a supported, stationary recording medium, such as paper or a transparency.
After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until an entire page is printed. In contrast, the page width recording apparatus includes a stationary printhead having a length sufficient to print across the width or length of a supported sheet of recording medium at a time. The supported recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process.
In ink-jet printing, it is also possible to create multicolor images on a print sheet. This type of printing may be used for full-color images, such as to reproduce a color photograph, or can be employed for "highlight" color, in which colored additions are made to a main portion of the image or text, which is typically black. In either case, the most common technique for color ink jet printing has been to sequentially image two or more colors, in separate printing steps, onto the single print sheet. This superimposition can be carried out in any number of ways. To take the example of a full-width apparatus printing black and one highlight color, an apparatus may print out the entire black portion of the desired highlight image on the sheet, and then recirculate the print sheet once again to image the highlight color portion of the image onto the same sheet from another printhead loaded with the colored ink; such a system has a serious disadvantage in the production of accurate registration of the composed images.
Alternately, two printheads may be positioned very close to each other, and render the two portions of the image onto the print sheet almost simultaneously, although two different areas of the print sheet will be printed upon by the different printheads at the same time or with a small time lag. For a full-color process image, four types of ink (yellow, magenta, cyan, and black) are emitted from four separate printheads during printing as the print sheet is moved relative to them.
Ink jet recording apparatus in which the printhead and the ink supply are combined into a single replaceable "ink jet cartridge" are well known, and so are ink jet recording apparatus in which the printhead and the ink supply usually is in the form of a replaceable ink tank. This invention relates to both types of recording apparatus. Usually, the manufacturing site for the replaceable ink cartridge or tank is located at a particular altitude over sea level having a particular typical atmospheric pressure. Each replaceable cartridge or tank is filled with sufficient ink and sealed, trapping usually some amount of air trapped within the cartridge or tank at the manufacturing site. Thereafter, the filled and sealed cartridge or tank may end up, and usually ends up, being installed and used in a recording apparatus that, for example, may be located several thousand feet above sea level greater than the manufacturing site. In such a case, some conventional installed ink tanks for example, are likely to, and have been found to be susceptible to leaking during installation.
It has been found that after filling the ink tank with liquid ink, and then sealing it, an air bubble is inevitably trapped in the liquid chamber at an initial internal pressure P1. At such time of sealing the tank, the pressure P1 is approximately equal to the external atmospheric pressure of the manufacturing site, resulting in an initial pressure differential between P1 and the atmospheric pressure. Thereafter, an expansion in the volume of the trapped air bubble can occur if its pressure P1 changes from high to low as is the case when the ink tank is sealed at a low altitude but is opened at a high altitude. In addition, temperature changes in accordance to Boyle's Law, can cause increases or decreases in the initial internal pressure P1, resulting in an actual and different internal pressure P3. Furthermore, in addition to significant changes in altitude or temperature, barometric fluctuations can increase or decrease the external atmospheric pressure, thereby resulting in an actual external pressure that is different from that of the manufacturing site. In either case, this creates an actual net pressure difference across the walls of the ink tank that is different from the initial pressure differential. This actual net pressure difference has been found to render the ink tank susceptible to leaking during installation given differences in altitude between a manufacturing site and the installation site.